-LRB- CNN -RRB- -- Syrian opposition forces may have executed as many as 30 people , most of them government soldiers , in rural Aleppo , according to the United Nations , which cited videos of the killings posted on the Internet in July .

U.N High Commissioner for Human Rights Navi Pillay called the allegations `` deeply shocking '' and called Friday for an independent investigation into the incident , which appears to have taken place in Khan al-Assal in northern Syria .

`` There needs to be a thorough independent investigation to establish whether war crimes have been committed , and those responsible for such crimes should be brought to justice , '' Pillay said in a statement .

The videos , posted to the Internet between July 22 and 26 , show government soldiers being ordered to lie on the ground , bodies being collected by doctors , corpses strewn along a wall and bodies in Khan al-Assal bearing gunshot wounds to the head .

Pillay 's office also has information that Syrian rebels are still holding government officers and soldiers prisoner , the statement said .

Reminding opposition forces that all captured or wounded soldiers must be treated in accordance with international law , Pillay said , `` Opposition forces should not think they are immune from prosecution . They must adhere to their responsibilities under international law . ''

The U.N. Independent International Commission of Inquiry on the Syrian Arab Republic has previously said both rebel groups and government forces and their affiliated militias are demonstrating `` emerging patterns of summary execution and murder . ''

News of the killings came as the Local Coordination Committees , an umbrella group for the opposition , said 19 of its loyalists had been killed , including two children . Most of the deaths came in Damascus , the LCC said .

In a speech to government forces Thursday -- Army Foundation Day -- President Bashar al-Assad said he was confident the government would triumph over the opposition , according to the state-run Syrian Arab News Agency .

'' Had we in Syria not been confident of victory , we would not have been able to remain steadfast and resist the aggression for over two years , '' the president said , according to SANA ,

The U.N. human rights office also said Friday that investigators have been allowed to visit rebel-controlled Khan al-Assal as part of a separate investigation into the alleged use of chemical weapons .

The rebels announced Friday they had written U.N. Secretary General Ban Ki-moon , requesting an investigation and demanding `` that any individuals found to be involved in the deployment of chemical weapons in Syria be held accountable for these crimes . ''

According to the United Nations , more than 100,000 people have been killed since fighting began . Almost 2 million have fled the country , and 4 million more have been internally displaced .

CNN 's Eliott C. McLaughlin contributed to this report .

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U.N. commissioner : Video shows rebels may have executed up to 30 people

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Office says it has information rebels may still be holding soldiers , others hostage

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Report comes as rebels urge U.N. to investigate use of chemical weapons

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About 100,000 killed , 6 million displaced since fighting began , U.N. says